1. Field of the Invention
The present invention relates to a motor for which low vibration, high-accuracy rotation, and high rigidity are required, and more particularly to improvements of a motor using a split-type core for a stator.
2. Description of the Related Art
In the processing of manufacturing semiconductor devices, a polishing apparatus is used for the purpose of planarizing the surface of a substrate or making a thickness thereof uniform. For example, a chemical mechanical polishing (CMP) apparatus is used to planarize a surface of a wafer which is used as a semiconductor substrate. In such a polishing apparatus, in order to effect very fine processing with a total thickness variation on the order of several 10 nm, low vibration, high-accuracy rotation and the like are required for a motor which is used for the polishing apparatus. In addition, the motor requires high rigidity to rotatively drive a turntable of the CMP apparatus, since the motor must withstand a large thrust load at the time when the weight of the turntable is applied to the motor
The motor shown in FIG. 17 shows an example of a dc brushless motor which can be used for the polishing apparatus (see FIG. 16 which will be referred to later).
As shown in FIG. 17, a brushless motor 100, if roughly classified, includes a rotor 10, a stator 20, a bearing unit 30, a housing 40, and a detector 50. It should be noted that a description of a control system for driving the motor will be omitted.
The rotor 10 for generating the rotational torque (power) includes a rotating shaft 11 which is hollow for passing the piping and the wiring therethrough as well as a magnet placing portion 12 which is a tubular member provided on a substantially central portion of the rotating shaft 11 and on the outer periphery of which a plurality of magnets 13 are arranged. The stator 20 is formed such that a plurality of split cores 23, which are cores 21 wound around with windings 22 as shown in FIG. 19, are arranged in an annular form as shown in FIG. 18. The cores 21 includes laminated silicon steel sheet. A yoke portion 23 holds the stator 20. The housing 40 includes a left housing 41, a right housing 42, a right-side cover 43, and so on. The left and right housings 41 and 42 clamp and hold the stator 20. In addition, the left and right housings 41 and 42 rotatably support the rotating shaft 11 via a left bearing 30a and a right bearing 30b of the bearing unit 30, which are respectively disposed on both sides of the magnet placing portion 12, such that the clearance between the pole face 13 of the rotor 10 and an end face of the stator core 21 opposing thereto assumes a predetermined air-gap length. The position detecting encoder 50 for detecting the rotational position of the rotor 10 is provided inside the cover 43. The position detecting encoder 50 includes a rotating disk 51 which is attached to the right-hand side of the rotating shaft 11 and on which markers are formed, a detecting head 52 for generating pulses corresponding to the markers, and so on. An output of the detecting head 52 is supplied to a motor controller, and is used for controlling a rotation of the motor. The motor controller controls a current supplied to the respective windings 22 to generate a rotating magnetic field, thereby causing torque to be produced in the magnets 12 and controlling the rotation of the motor.
The brushless motor 100 is used in the polishing apparatus, as shown in FIG. 16 to be referred to later. A table 110 is fixed to mounting holes (threaded holes) in the rotor 10 by means of bolts. Further, a frame 120 is fixed to mounting holes (threaded holes) in the housing 41. In consequence, the motor 100 turns the table 110 in response to a drive current supplied from the controller.
Further, to facilitate the assembly and facilitate a winding of magnetic field windings and for other similar purposes, there is a case where the core of the stator is formed in a split type. If the winding of coils is effected in a state in which the core is split, alignment winding is possible, so that a multiplicity of windings is made possible. In addition, since the slot opening can be minimized, this arrangement is advantageous in terms of the magnetic circuit, and it is possible to obtain a motor with improved efficiency.
For example, a motor such as the CMP apparatus needs to withstand the large thrust load and needs to high-precisely rotate with a low oscillation and high out-put power. However, with the above-described structure of the motor, the overall arrangement of the motor tends to be elongated in the axial direction. In addition, if an attempt is made to secure a required torque and the like, the radial dimension cannot be made as small as expected. This makes it difficult to make the miniaturization of the apparatus using the motor difficult.
Further, since both sides of the rotating shaft are supported by the bearings of the housing, the structure adopted is such that the heat generated in the winding portions is trapped inside the housing, so that effective dissipation of the heat is difficult.
Further, more if there are variations and the like in the accuracy of parts, when the split cores of the stator provided with the windings are assembled into a cylindrical shape, the split cores cannot be assembled such that the inner and outer peripheries of the stator become completely round. Although the stator is built into the tubular yoke by press fitting or the like, it is not easy to press the stator which is not correctly completely round into the yoke, which can cause a difficulty in assembly and a hindrance.